Carbon-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 97

Table 6.1. Continued

Radical pK a Parent com-

pound

∆ pK a Reference

2.8 Maleimide* − Hayon and Simic (1972)

von Sonntag et al. (2002)

This general phenomenon is also of some importance in the case of the proton-
ation of radical anions which can protonate at a heteroatom or at carbon. Kineti-
cally, protonation at the heteroatom is always faster even when protonation at
carbon is thermodynamically favored. A case in point is the protonation of the
Thy radical anion (Chap. 10.4).
α

-Hydroxyalkyl radicals are less solvated than their parent alcohols due to a
lower polarity of the C O bond and lower H-bond acceptor ability of the oxygen

atom (Block et al. 1999). They are considerably more acidic than their parent
alcohols by four or more p K units (Table 6.1). The same effect is observed even
more dramatically the case of the cyclic dipeptides (Merenyi et al. 1986; Mieden
and von Sonntag 1989). This increase in acidity is possibly largely due to an in-
crease in mesomery such as shown in reaction (8), and captodative substituent
effects (Viehe et al. 1985) seem to contribute further in the case of the peptide
radicals.

−

Interestingly, the p K a values of acetic acid and its corresponding radical are
practically identical (Table 6.1; Neta et al. 1969; Schuchmann et al. 1989, see also
Schuchmann et al. 2000). In this case, not only the anion, but also the acid expe-
riences a gain in mesomery [reaction (9)].

There is a continuing discussion as to the p K a value of • CO 2 H radical. In the lit-
erature, values are found ranging from -0.2 (Jeevarajan et al. 1990), 1.4 (Buxton
and Sellers 1973), 2.3 (Flyunt et al. 2001) to 3.9 (Fojtik et al. 1970). Why the value